1. Field of the Invention
The present invention relates to telephone trunk circuits and more particularly to an AC supervisory signal detector circuit for use in a digital private automatic branch exchange, to detect AC supervisory signals supplied by a central office. The present circuit designed for inclusion in a PABX trunk circuit, provides an indication to alert a called station of an incoming call.
2. Description of the Prior Art
In a telephone system it is necessary that a central office provide AC supervisory signals via a trunk circuit to a PABX. For example, these AC signals indicate to the PABX that it is about to receive an incoming call and that the central office requires recognition of the reception of the incoming call. This signaling is accomplished via application of a low frequency AC voltage superimposed upon a DC bias voltage of either polarity applied at the central office to the tip and ring leads of the trunk circuit.
Existing trunk circuits typically sense the application of such ringing voltages by the use of circuits specifically tuned to the frequency of the AC voltage to be detected and the polarity of the DC bias voltage. A disadvantage of these types of circuits is that a different circuit must be used for the detection of each frequency of AC supervisory signal which is applied.
A solution of these problems is taught by U.S. Pat. No. 3,849,605 issued to S. L. Russell, on Nov. 19, 1974, which demonstrates the use of a high voltage diode bridge rectifier circuit and an optical coupler including a light emitting diode and single photosensitive transistor. The output of this configuration requires latching in order for the detection of the continuity check level. Furthermore, the solid state continuity check circuit described in the Russell patent must be switched into and out of the tip and ring loop in order to avoid distortion during voice transmission thereby requiring relay contact control. Furthermore, this circuit exhibits another disadvantage which is the required use of high voltage diodes for the bridge circuit.
Therefore, it is an objective of the present invention to provide an AC supervisory signal detector circuit for detecting ringing type signals transmitted at a central office to a PABX in which the detector circuit is not required to be switched into and out of the tip and ring loop; the output of the detector circuit is impervious to spurious signals such as lightning or interruption by loop signaling of dialing; and the output does not require latching. The present invention also provides detection of multiple frequencies by the use of a single detector circuit.